A head/disk tester is an instrument that is used for testing the characteristics of magnetic heads and disks such as a signal-to-noise ratio, track profile, etc. The tester should simulate those motions of the head with respect to the disk that occur in an actual hard disk drive during operation. Each tester consists of two components; a mechanical component, commonly referred to as a spinstand, that performs movements of the head with respect to the disk, and an electronic component that is responsible for measurement, calculation, and analysis of the measured signal. Test results depend on the accuracy of both components.
A typical prior art spinstand for a head and disk tester is described in U.S. Pat. No. 4,902,971, issued in 1990 and incorporated herein by reference. It includes a stationary base plate which supports a stepper motor. The stepper motor includes an output shaft which is connected to a coaxially-arranged lead screw. The lead screw rotates in bearings and engages a nut which is rigidly fixed to a linear slide. In this manner, motor rotation is converted into linear movement of the slide. The slide supports an arm upon which a magnetic head is rigidly mounted. The arm can be rotated relative to the slide so as to provide angular movement of the head. In this manner, the tester provides a range of radial movements of the head and variations in skew angle, where skew angle is the angle between the longitudinal axis of the magnetic head and a line extending tangentially from a concentric track of the magnetic disk.
As the density of magnetic recording increases, additional information tracks are compressed into a given disk area. The decrease in track size heightens the demand for improved accuracy in head positioning. This challenge is compounded by a desire for faster operation speed, and low cost for tester manufacture and tester operation.
Attempts have been made to improve accuracy in magnetic head positioning by providing a head and disk tester with separate coarse and fine-positioning mechanisms. Such a tester is disclosed in pending U.S. patent application Ser. No. 08/813,345 filed on Mar. 7, 1997 by the same applicant and incorporated herein by reference. In this tester, the coarse-positioning mechanism is the same as the conventional positioning mechanism described above. A fragmental side view of the fine-positioning mechanism is shown in Prior Art FIG. 1. This embodiment includes a slide 12 supportive of a nut 14 adapted to engage with lead screw 10, and a deformable body 16, e.g., in the form of a deformable parallelepiped. The lower surface 17 of deformable body 16 is rigidly attached to the aforementioned slide 12. The upper surface 19 of deformable body 16 supports a magnetic read/write head H. A piezo actuator 20 is placed between a first side of the deformable body 16 and a post 18 stationary with respect to slide 12. Piezo actuator 20 is preloaded between post 18 and deformable body by a set of springs 24 which urge the deformable body toward the piezo actuator 16 and the post 18.
The positioning operation consists of two steps. Coarse positioning is achieved by inducing rotating lead screw 10 by stepper motor (not shown), causing linear movement of slide 12, together with deformable body 16 and magnetic head H to the vicinity of a predetermined position. Upon completion of coarse positioning, a fine positioning mechanism is activated by piezo actuator 20 to position deformable body 16 and magnetic read/write head H within the specified accuracy of the tester. The time required for placement of the magnetic read/write head H within a specified tolerance is referred to as the settling time of the tester. Efficient testing procedures require that settling time be as short as possible.
Many critical magnetic head and disk tests, for example measurement of the read-write offset of magneto-resistive (MR) read/write heads, or track profile measurement, require the magnetic read/write head to be repeatedly repositioned at different small positional offsets from its original location. In most cases, these small changes in the position of the head can be achieved using the fine positioning mechanism only, without activation of the coarse positioner. Since it is desirable to minimize test duration, each fine positioning operation should be performed as expeditiously as possible.
It is important to note, however, that lead screw 10, slide 12, piezo actuator 20, post 18, deformable body 16, etc., together form a complex mechanical system having natural resonance frequencies. If the stepper motor of the coarse positioner moves slide 12 too quickly during coarse positioning, or if piezo actuator 20 is driven too fast during fine positioning, the time for head positioning decreases, but excessive oscillations arise in this system, resulting in an increase in settling time.
An optimal response time of the piezo actuator corresponds to the minimum settling time for a given positioning accuracy. This response time is a function of the mass and stiffness of slide 12, the mass of deformable body 16, the stiffness of piezo actuator 20, the stiffness of the post 18, the stiffness of lead-screw nut 14, and the stiffness of the lead screw support unit (not shown). The smaller the respective masses, and the higher the stiffnesses, the faster piezo actuator 20 can be driven to achieve the minimum settling time.
The mass of the slide 12 cannot be decreased below some minimum because certain baseline mechanical configurations are required in order to mount lead-screw nut 14 and piezo actuator 20 on the slide 12 properly, while maintaining slide 12 stiffness. Likewise, the mass of the deformable body 16 can be decreased only to some minimum since certain mechanical configurations are required in order to mount the magnetic read/write head H properly on the deformable body 16. Due to limitations including part size, material stiffness and availability of commercial parts of higher stiffness, increasing stiffness beyond a certain amount is not possible, either. In other words, the conventional fine-positioning mechanisms of head/disk testers have inherent dynamic characteristics that limit further increase in testing speed.
The prior art fine positioning system described above is therefore subject to vibrations, because the reaction R from the force F applied by piezo actuator 20 to deformable body 16 will be transmitted through post 18, slide 12, and nut 14 to lead screw 10. At each positioning operation, the aforementioned system will oscillate, and therefore, measurement cannot be initiated until the system is settled down to a substantially stable condition.